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Case Study: Fluid Contacts Defined on Norwegian Continental Shelf



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"A new generation of sampling technology has been launched that will change the future of downhole sampling. Close to zero contamination sampling is not only important for general fluid analysis (improved quality laboratory samples), it is also a significant step toward achieving the goal of doing high-quality downhole fluid measurements without being affected by mud filtrate contamination."
Kåre Otto Eriksen, Special Advisor on Formation Sampling, Statoil



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The top track shows the commingled flow split by increasing the guard flowline pump rate. In the bottom track, the GOR confirms the stabilization of low-contamination focused flow.


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At these two stations only 0.5 m apart, the pure fluid obtained with the Quicksilver Probe tool at X,X46.0 m was determined by downhole fluid analysis to be black oil, whereas at the station at X,X45.5 m, the compositional analysis indicated dry gas.

Downhole fluid analysis of Quicksilver Probe representative samples accurately characterizes reservoir fluid composition and places contacts


Challenge

Accurately and fully characterize fluid samples to better understand the reservoir and guide facilities design and production planning.

Solution

Collect ultralow-contamination fluids with Quicksilver Probe focused fluid extraction for spectroscopic and CFA* Composition Fluid Analyzer analysis downhole.

Results

Definition of the gas/oil contact within 0.5 m [1.6 ft] was more accurate than what could be achieved using pretest gradients in the well.

Looking for answers without a production test

Because of environmental restrictions, a production test had not been planned by Statoil for the final appraisal well before field development on the Norwegian Continental Shelf. Therefore, full, accurate characterization of representative samples was critical for guiding the material selection of pipeline and surface facilities, process design, and production planning. To aid in the collection of representative hydrocarbon fluids, the well was drilled with water-base mud to avoid miscible contamination.

Collecting ultralow-contamination fluids for downhole analysis

The ultralow-contamination fluids delivered by the Quicksilver Probe technique optimize downhole and laboratory fluid analysis, and the quick cleanup times add further efficiency. For example, pumping at one of the Quicksilver Probe stations—in a relatively tight zone with 17-mD/cP mobility—began with typically commingled flow through the guard flowline and then through the fluid-acquisition flowline (shown on the top plot of the flow rate). After 1,300 s, the pumping rate for the guard flowline was increased to divert contamination. The GOR in the bottom plot stabilized at about 2,300 s, indicating that the fluid in the acquisition flowline was clean. A sample with no detectable water-base mud was then collected as pumping continued at 2,800 s. The spikes in the GOR curve indicate the presence of produced formation fines, whichwas subsequently confirmed by laboratory analysis. A total of 19 sample chambers were filled from many levels in this one run of the Quicksilver Probe tool in the well.

Understanding the reservoir

Downhole fluid analysis is a powerful technique for characterizing fluids downhole because it avoids the potential changes in fluid composition that occur when samples are brought to the surface for analysis. The spectroscopic analysis used to identify pure fluid in the flowline is also used to characterize the fluid composition for downhole analysis in terms of methane (C1), ethane to pentane (C2 to C5), and hexane and heavier components (C6+).

Measurements taken at 16 stations further quantified the reservoir fluid composition and delineated fluid contacts, as shown in the plots made by the CFA Composition Fluid Analyzer module in the toolstring. At two fluid-scanning stations only 0.5 m [1.6 ft] apart, the lower station at X,X46.0 m indicated black oil with an apparent density of 0.868 g/cm3 and a low GOR of 133 m3/m3 [740 ft3/bbl]. At the upper station, at X,X45.5 m, the fluid composition showed dry gas, with an apparent density of 0.126 g/cm3 and C1 content greater than 91% of the mass. This definition of the gas/oil contact within 0.5 m was at a better accuracy than achieved using pretest gradients in the well.


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